The invention relates to an internal combustion engine exhaust gas system.
Present and future emission legislation on vehicle exhaust gas emissions sets considerable requirements on converting e.g., nitrogen oxides (NOx), hydrocarbons (HC), carbon monoxide (CO) and particulates into less hazardous compounds.
Exhaust gas aftertreatment systems generally involves a catalytic converter that needs to operate within a certain temperature interval. If the temperature is too high the converter may be deactivated or even destroyed, and if the temperature is too low the reaction rate of the desired catalytic reactions is too low. Below the so-called light-off temperature of the catalyst the reaction rate is close to zero.
Various catalytic systems exist for e.g., oxidation of HC and CO, for regeneration of particulate filters, and for reduction of NOx. An example of the latter is selective catalytic reduction (SCR) where urea/ammonia or HC is used to reduce NOx into nitrogen. In such a system urea or hydrocarbon is injected into the exhaust by an injector and the NOx is reduced to nitrogen over the catalyst. Another example is lean NOx adsorber/trap (LNA, LNT) where NOx is trapped in an adsorber during normal lean operation and reduced to nitrogen during short periods of rich operation of the engine.
In certain modes of operation, such as low load situations, and in certain applications, such as the use of a turbo system in connection to a diesel engine, the temperature of the exhaust gas has a tendency to fall below the suitable temperature interval of the converter. This results in a decreased conversion of the hazardous compounds. In order to solve the problem with low-temperature exhaust gas, large efforts have been made on developing catalysts with lower light-off temperatures. A general difficulty in this regard is that a lower light-off temperature normally gives a lower deactivation temperature. Another approach has been to focus on the location of the catalytic converter; e.g., to place the converter very close to the engine. Problems associated with this approach are increased risk of thermal deactivation and constraints in the freedom of placing the converter. Still another approach has been to modify the combustion as to keep up the exhaust gas temperature. A general drawback with this approach is that it results in an increased fuel consumption.
The invention concerns an internal combustion engine exhaust gas system, comprising an EGR arrangement adapted to lead a first flow of exhaust gas (the EGR flow) from an outlet side of the engine to an inlet side of the engine, an exhaust gas conduit adapted to lead away a second flow of exhaust gas (the exhaust flow) from the outlet side of the engine, and an energy recovering unit, such as a turbo, associated with the exhaust gas conduit, the unit being adapted to recover exhaust gas energy from the exhaust flow. The invention is characterized in that the system comprises a heat exchanger adapted to allow heat exchange between at least a part of the EGR flow and at least a part of the exhaust flow, the heat exchanger being associated with the exhaust gas conduit at a position downstream the energy recovering unit.
The use of an energy recovering unit, such as a turbo, results in a temperature decrease of the exhaust gas, i.e. the temperature of the exhaust flow downstream the energy recovering unit will be lower than the temperature of the EGR flow. By providing the system with a heat exchanger this temperature difference can be utilized by transferring heat from the EGR flow to the exhaust flow. An advantageous effect of the characterizing feature is thus that the temperature of the exhaust gas can be increased in order to enhance the efficiency of the exhaust gas aftertreatment. For instance, if the temperature of the exhaust gas is below the suitable temperature interval of a catalytic converter the present invention makes it possible to increase the exhaust gas temperature to above the lower limit of the suitable interval. A further advantageous effect of the present invention is that the heat exchanger at the same time works as an EGR cooler. In many applications the EGR flow needs to be cooled in order to reduce the amounts of NOx produced during combustion. This is conventionally done in a separate EGR cooling unit. Thus, the present invention reduces the need for such separate cooling units.